Many photographically useful agents may be advantageously incorporated in a photographic light-sensitive material to aid, for example, in processing, and they typically differ from those which are used as additives in a processing solution. Their incorporation in a photographic light-sensitive material enables effective utilization of photographic agents which tend to decompose under acidic, alkaline or oxidation-reduction conditions, and consequently, cannot withstand prolonged storage in a processing bath. At the same time, it is possible to simplify the composition of the processing solution used and to facilitate the preparation of the processing solution. In addition, it is possible to provide required photographic agent which functions only at a desired time during the photographic processing or only in a specified layer and the neighboring layers of a multilayer photographic light-sensitive material. Furthermore, the amount of photographic agent released from the precursor in the photographic light-sensitive material can be varied depending on the extent of silver halide development. However, if a photographic agent is added to a photographic light-sensitive material in its active form, the photographic agent does not exhibit the expected degree of activity because during storage before photographic processing, it reacts with other components contained in the photographic light-sensitive material or it is decomposed by heat, oxygen and so on.
One method for solving this problem involves adding a photographic agent to a photographic light-sensitive material in the form of its precursor i.e., in a form such that its active group is blocked and rendered photographically inactive. This method has various advantages in different applications. For instance, when the useful photographic agent is a dye, blocking a functional group of the dye which has a significant effect on its spectral absorption characteristic results in a shift of its spectral absorption band to shorter wavelengths or to longer wavelengths. Therefore, even if the dye is present in a silver halide emulsion layer having a spectral sensitivity in the wavelength region corresponding to the absorption band of the dye in the unblocked state, a lowering of the sensitivity due to the "filter effect" can be prevented.
Where the useful photographic agents are antifoggants or development restrainers, blocking of their active groups makes it possible to suppress desensitization arising from adsorption of these agents to light-sensitive silver halide grains and formation of silver salts during storage. At the same time, release of these agents when required permits the reduction of fog density without an accompanying decrease in sensitivity, along with the prevention of fog due to overdevelopment, stopping development at a desired time, and so on.
In still another case where the useful photographic agents are developing agents, auxiliary developing agents or fogging agents, if their active or adsorptive groups are blocked, various photographically adverse effects which arise from semiquinones and oxidants produced by air oxidation upon storage can be prevented. Generation of fogging nuclei upon storage can also be prevented because injection of electrons into the silver halide grains can be inhibited, resulting in stable processing.
Where the useful photographic agent is a bleach accelerating agent or a bleach-fix accelerating agent, it is possible to prevent reactions with other components also present in the photographic light-sensitive material during storage by blocking its active group, and to fully activate the agent at a desired time by removing the blocking group during processing.
In the present invention the above-described "active group", "functional group" and "adsorptive group" are generally referred to as the "active group".
As described above, photographic agent precursors can be extremely valuable in playing photographic agents to their best advantage. However, such precursors must satisfy two very severe requirements in practical use, that are contradictory to each other; one is ensuring stability the precursor during a storage condition, and the other is removing its blocking group at a desired time during processing and thereby releasing the photographic agent rapidly and efficiently.
A number of techniques for blocking a photographic agent are known. For instance, a technique using a blocking group such as an acyl group, a sulfonyl group or the like is described in Japanese Patent Publication No. 48805/72 (corresponding to U.S. Pat. No. 3,615,617); one which utilizes such blocking groups to release a photographic agent by the so-called reversal Michael reaction is described in Japanese Patent Publication Nos. 39727/79, 9696/80 and 34927/80 (corresponding to U.S. Pat. Nos. 3,674,478, 3,791,830 and 4,009,029, respectively); one which utilizes such a blocking group to release a photographic agent with the production of quinone methide or its analogs by intramolecular electron transfer is desired in Japanese Patent Publication No. 39727/79 (corresponding to U.S. Pat. No. 3,674,478), Japanese Patent Application (OPI) Nos. 135944/82 (corresponding to U.S. Pat. No. 4,416,977), 135945/82 (corresponding to U.S. Pat. No. 4,420,554) and 136640/82 (corresponding to U.S. Pat. No. 4,420,554) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"); one which utilizes an intramolecular ring-closing reaction is described in Japanese Patent Application (OPI) No. 53330/80 (corresponding to U.S. Pat. No. 4,310,612); one which utilizes cleavage of a 5-membered or 6-membered ring is described in Japanese Patent Application (OPI) Nos. 76541/82 (corresponding to U.S. Pat. No. 4,335,200), 135949/82 (corresponding to U.S. Pat. No. 4,350,752) and 179842/82: and so on. However, these photographic agents blocked with known blocking groups have several disadvantages. For example, although stable during storage, some precursors require a highly alkaline condition, such as a pH higher than 12, for processing because the photographic agent-releasing rate thereof is too slow. On the other hand, some precursors decompose gradually, losing their function as precursors during storage, even though they can release the photographic agent at a sufficiently fast rate when processed under mild conditions, such as a pH range of 9 to 12.
These defects are considered to be a result of the fact that release of photographically useful agents from the blocked photographic agents relies upon the attack of OH.sup.- ions. The ratio of OH.sup.- ion concentration between storage at a pH of from 6 to 7 and processing at a pH of from 9 to 12 at which conventional photographic light-sensitive materials are developed is from 10.sup.2 to 10.sup.6. Thus it is assumed that, for example, blocked photographic agents capable of releasing photographically useful agents at a half-life period of 3 minutes (i.e., 3 minutes is required for the decomposition of one-half the amount added) when processed at a pH of 10, will undergo decomposition at a half-life period of about 500 hours (3 min.times.10.sup.4) if stored at pH 6. This means that one half of the precursor added decomposes after about a 3-week storage period. Such blocked photographic agents are of no practical use at all. Further, blocked compounds which release photographically useful agents at a half-life period of 3 minutes when processed at a pH of 11, will have a half-life period during storage of about 30 weeks (10 times the above half-life period). This value is still unsatisfactory, and such precursors can be said to be unsuitable for practical use from the viewpoint of storage stability.
Another problem ascribable to the fact that the release of photographically useful agents from blocked photographic agents relies upon the attack of OH.sup.- ions is a reduction in the efficiency of releasing the photographically useful agents. That is, simultaneously with the release of the photographically useful agents, the action of OH.sup.- ions causes unexpected side-reactions, destroying the blocked photographic agents prior to the release of the photographically useful agents. This is reported in, for example, U.S. Pat. No. 4,135,929, Yuki Gosei Kyokai Shi (Journal of Synthetic Organic Chemistry, Japan), Vol. 39, page 331 (1981) and ibid, Vol. 40, page 176 (1982). This reduction in the releasing efficiency leads to an increase in the amount of blocked photographic agents required. Furthermore, in some cases, by-products resulting from the unexpected side-reactions adversely affect the photographic characteristics, making it unsuitable to use such blocked photographic agents for the expected purposes.
On the other hand, photographic usefulness of antifoggant precursors is known based on the observation that the sensitivity/fog ratio is greatly increased by a method in which antifoggants function on the way of the development step in conventional color development processing in comparison with a method in which the antifoggants function from the beginning of the development step. The term "on the way of the development step" means a step in which a development at exposure area attains to a late stage (i.e., an amount of the developed silver is increased) but fog is at an induction period yet (i.e., no fog is generated yet). The terms "Induction period" and "late stage" mean so-called an allover induction period and a late stage, respectively, which are limited in C. E. K. Mees and T. H. James, The Theory of Photographic Process, 3rd ed., pp. 352-353 (1966). Furthermore, it is disclosed in U.S. patent application Ser. No. 564,659 that precursor compounds of pyrazolidone type contribute to acceleration of development and an increase in sensitivity.